1. Field of the Invention
The present invention relates to a toroidal coil device used in, for example, radio receivers for automobiles which blocks AM signals and further relates to a method for manufacturing the toroidal coil device.
2. Prior Art
FIG. 11 illustrates a circuit diagram of a radio receiver used in automobiles.
In FIG. 11 an antenna 1 made of a conductive film is mounted on the rear windshield of an automobile. The feeding terminal of this antenna 1 is connected to input terminals 2a and 2b of the antenna, and to the input terminals 2a and 2b, an FM circuit 3 and an AM circuit 4 are connected respectively. Provided between the input terminals 2a and 2b and the AM circuit 4 is a choke coil 5 which blocks the FM signals. Also, between the connecting point of the choke coil 5 and the AM circuit 4 and a power source circuit 6, a toroidal coil 7 which blocks the AM signals is provided. Usuallu, the toroidal coil 7 is manufactured by an automatic winding machine which winds the conductive wires around a ring-shaped toroidal core. The toroidal coil may also be manufactured manually by winding the conductive wires around a ring-shaped core.
When a current as high as 20.varies.30 amperes flows through the thus manufactured toroidal coil, the conductive wire used therein coil must have a very large diameter. However, when wire of a lesser specific diameter is used, it is very difficult, if not impossible, to wind the wire around the core accurately. As a result, it is very likely that the wire will be wound unevenly during the manufacturing process.
As mentioned above, a large amount of current may occasionally flow through the toroidal coil. Also in order to cancel magnetic flux in the toroidal coil, current may flow in the opposite direction through the coil. Accordingly, when the way in which the wire is wound is varied even a little, the core may become saturated. Thus, a toroidal coil with high precision wire winding is very much desired.